Coordination compounds cerium

Cerium(III) tris(/3-diketonate) complexes are readily oxidized to the cerium(IV) compounds. They are volatile with vapor pressures high enough for MOCVD use. A number of diketonates are now better characterised, prompted by the possibility of using them as CVD materials and petrol additives, as well as a source of cerium oxide as an oxygen store for catalytic converters. [Ce Me3CCOCHCOCMe2(OMe) 4], Ce(acac)4 (Figure 32), Ce(dbm)4, [Ce(pmhd)4], and Ce(tmhd)4 all have square-antiprismatic coordination of cerium (in the [Ce(catecholate)4]" ion the coordination is dodecahedral). 

The lanthanoids are sometimes simply called the rare earths, although strictly the earths are their oxides. Nor are they particularly rare they occur widely, usually together. All are silvery very reactive metais. The/electrons do not penetrate to the outer part of the atom and there is no /-orbitai participation in bonding (unlike the d-orbitis of the main transition elements) and the elements form tew coordination compounds. The main compounds contain ions. Cerium also has the highly oxidizing Ce ... 

This chapter gives an overview on the chemistry of tetravalent lanthanide compounds, especially those of tetravalent cerium. Following a brief introduction, it covers the tetrahalides, dioxides, and other lanthanides(IV) salts. Coordination compounds of cerium in the oxidation state +4 include halogeno complexes and complexes of oxo acids, /3-diketonates and related Schiff-base complexes, as well as porphyrinates and related complexes. 

Cerium tetrakis(acetylacetonate), a venerable compound,695 exists as a and /3 modifications. Both these contain [Ce(acac)4] monomers of D2 square antiprismatic coordination but differ in the crystal packing. In the a form,696 Ce—0 = 2.36-2.43 A with the ring angle O—Ce— 0 = 72°, while in the / form697 Ce—0 = 2.32 A and O—Ce—0 = 71.3°. The tetrakis(acetyl-acetonates) of Zr, Hf, Th, U and Pu also show dimorphism. The tetrakis(dibenzoylmethanate) [Ce(dbm)4] adopts a similar square antiprismatic structure with Ce—O = 2.299-2.363 A and O—Ce—O = 70.8° or 71,2°.698 There is some tendency for Ce3+ /J-diketonates to pass into the Ce4+ tetrakis(/3-diketonate). A series of tri- and tetra-valent cerium /3-diketonates has been examined from the point of view of the effect of additional ligands such as Ph3PO on this process, and it was found that Ce3+ /J-diketonates were stabilized by adduct formation, particularly by 1,10-phenanthroline.699... 

Th(PhCOCHCOPh)4 has been reported as being isomorphous with the corresponding protactinium(IV), uraniumflV) and cerium(IV) complexes the coordination geometry in the last is a triangular faced dodecahedron, but a more recent publication73 reports the coordination geometry of the uranium(IV) compound as square antiprismatic. 

The coordination geometry in anhydrous Th(CF3COCHCOMe)4 is a 1111 ( )4-422) antiprism 74 the structure of the monohydrate has been discussed earlier (p. 1144). Th[CF3COCHCO(2-C4H3S)]4 is isostructural with the cerium(IV), uranium(IV) and plutonium(IV) analogues. The coordination polyhedron is a distorted dodecahedron in which the four ligands span the two perpendicular trapezoids of the dodecahedron.75 In the complexes M(n-C3F7COCHCOBut)4, the thorium(IV), uranium(IV) and neptunium(IV) compounds are isomorphous, but the plutonium compound is not. 

This is the case with fluoro complexes and the crystal structure of (NH CeFd shows the coordination is nearly square antiprismatic with cerium having eight fold coordination [140], However the compounds Cs2NaMF6 and analogous chloride have octahedral coordination [139]. Raman spectra of the chloro compounds of La, Pr, Nd and Er have been obtained [146-148]. Raman spectra along with the far infrared spectra of Nd and Er show the octahedral coordination of the lanthanide ion. For these complexes the location of Aig band is as follows La = 274 cm-1 Pr = 281 cm-1 Nd = 278 cm-1 Er = 289 cm"1. 

The only complexes of lanthanum or cerium to be described are [La(terpy)3][C104]3 175) and Ce(terpy)Cl3 H20 411). The lanthanum compound is a 1 3 electrolyte in MeCN or MeN02, and is almost certainly a nine-coordinate mononuclear species the structure of the cerium compound is not known with any certainty. A number of workers have reported hydrated 1 1 complexes of terpy with praseodymium chloride 376,411,438), and the complex PrCl3(terpy)-8H20 has been structurally characterized 376). The metal is in nine-coordinate monocapped square-antiprismatic [Pr(terpy)Cl(H20)5] cations (Fig. 24). Complexes with a 1 1 stoichiometry have also been described for neodymium 33, 409, 411, 413, 417), samarium 33, 411, 412), europium 33, 316, 411, 414, 417), gadolinium 33, 411), terbium 316, 410, 414), dysprosium 33, 410, 412), holmium 33, 410), erbium 33, 410, 417), thulium 410, 412), and ytterbium 410). The 1 2 stoichiometry has only been observed with the later lanthanides, europium 33, 411, 414), gadolinium, dysprosium, and erbium 33). 

The aUcoxides Ce(OR)4 are another well-developed class of Ce compounds. The aUcoxide group plays an important role in stabilising the Ce(lV) state, witness the existence of [CeCp2(OBu )2] and [CeCp3(OBu )], when other Ce(lV) organometalhcs cannot be isolated. The isopropoxide is volatile in vacuo below 200 °C (its bis(isopropanol) adduct has six-coordinate cerium). General methods are available to... 

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